With constant improvement in living standards, it has become increasingly common to utilize air conditioning technology to provide cooling sources in summer and heating sources in winter. Normally, the electricity use on air conditioning is light at night but heavy during the day, which competes with other usage of electricity at peak hours while capitulating at trough hours. This is a major factor causing the peak-trough differential usage in the electricity network. To ensure safety and reasonable and efficient operation of the electricity network, electricity is set at different prices for peak and trough hours to encourage the shifting of electricity usage from peak to trough hours.
Thermal (heat or cool) storage is a technique to adjust electricity usage under the peak-trough differential pricing regime. It stores “heat” or “cool” produced during night when the electricity price is relatively low, and then releases such stored thermal energy at a time when electricity is more expensive, thus achieving the double benefit of leveling the peak to fill the trough in electricity usage and cost-saving for electricity users.
Thermal charging and releasing is the core of this technology. Water thermal storage technology uses water as a thermal storage medium to store thermal energy utilizing the absorption and releasing of sensible heat during changes of the water temperature.
The natural stratification water thermal storage technology makes natural stratification of hot and cold water in the same tank with a very simple structure and without artificial separation devices, by utilizing the principle that water of higher temperatures floats upward with a lower density whereas water of lower temperatures tanks with a higher density. The core of water thermal storage technology is to prohibit or inhibit the mixing and heat exchange of stored water of different temperatures. Since the coefficient of thermal conductivity of water is relatively small, as long as stratification is stable, heat exchange will be kept relatively small. The key to maintain the stability of stratification is to ensure that the diffusers of hot and cold water ports can control water flows slowly and evenly into water thermal storage tanks in the form of a density flow. Apparently, the bigger water flow is, the stronger is water distribution intensity (flow volume per area of water distribution) the easier is to disturb water in tanks and mix hot and cold water together, resulting in lower thermal storage efficiency.
In order to enhance the thermal storage capability of an air conditioning system, it is generally desirable to have a tank with a sufficient volume capacity. However, in practice, when it is impossible to construct a tank with a sufficient volume capacity due to certain constrains, a system with multiple tanks is instead utilized. There are two solutions for this in the prior art:
1. Each tank stores thermal energy individually. For example, when storing “cool” (cold water), the first tank is charged with cool, and then the second tank, and go on in this order; the same way applies when releasing cool. There are two disadvantages with this method: 1) when switching tanks, it requires frequent operations which may easily cause mistakes; 2) for each single tank, the power of heat or cool sources is relatively high, and flow is relatively big if the system is fully loaded, resulting in undesirable efficacy in water diffusion and efficiency in thermal storage; however, if lowering the power of operation, electrical efficiency of cool (hot) source will also be compromised.
2. Tanks are connected in series: e.g. the hot port of the first tank is connected to the cold port of the second tank, the hot port of the second tank is connected to the cold port of the third tank, and so on and so forth in this order. When charging cool, cold water flows into the first tank via its cold port, warm water flows out of the last tank via its hot port, and the cool charging process is completed when cold water fills all the tanks. In this way, there are three disadvantages: 1) the flow may be relatively high to each tank in series, causing an undesirable efficacy in water diffusion. 2) the flow goes through the water diffusers of each tank several times, which increases the possibility of mixing hot and cold water and reduces significantly the efficiency of thermal storage; 3) if each serial-connected tank is a open system, the flow between the tanks must be driven by the water level differential, e.g. the water level of the upper tank must be higher than that of the lower tank. The flow directions in the processes of charging cool (heat) and releasing cool (heat), however, are completely different, requiring opposite water gradients of tanks, which results in water levels of tanks at each end having quite big difference in such two processes, as well as additional difficulty in the design of water diffusers and construction, and waste of storage space; if each serial-connected tank is a closed system, each tank must be pressurized, thus increasing the cost.
In the prior art, there is no report that tanks are connected in parallel to store thermal energy simultaneously or synchronously, because:
1. Each tank is an open, non-pressurized container with its free surface in contact with air. The thermal storage system with many parallel-connected tanks is thus a complex open fluid system with many free surfaces in contact with air. In the process of charging or releasing thermal energy, due to various pressure drops produced by the flow passing water diffusers of each tank, the height of the free surface in each tank will significantly change. If the water level is too high, the fluid will overflow. If the water level is too low, the upper diffuser will be exposed to air, causing pump suction problem. To ensure proper function, the water level of each tank must avoid intensive fluctuation during the synchronous operation of parallel-connected tanks.
2. During the synchronous operation of parallel-connected tanks, if tanks cannot complete thermal energy charging and releasing strictly synchronously, the volume utilization rate of tanks will be compromised. Taking cool storage as an example, if charging cool, the cold water out of the hot port of the tank that is the first to complete cool charging will cause alert at the chiller and shuts it down due the temperature of input water being too low. This will disrupts cool charging process for other tanks which fall behind, resulting in waste of volume of charging cool; if releasing cool, the hot water out of the cold port of the tank that is the first to complete cool releasing will cause the temperature in the cool supply tube to dramatically increase and the cool releasing is forced to stop, and then the tank, the process of which falling behind, cannot complete the process of releasing cool, which results in the waste of stored cool. In order to maintain the water temperature in the outputs and improve the efficiency of thermal storage, the tanks must be charged with or release thermal energy synchronously in the process of thermal energy charging and releasing.
The above mentioned problems have not been solved by prior art, thus the method of synchronous thermal energy charging and releasing by parallel-connected tanks still cannot be applied in practice.